In comparison with other optical sensors, which measure the intensity or phase of the light, frequency-proportional optical sensors are distinguished by being less distorted by ambient factors. In frequency-proportional optical sensors, the radiation source is as a rule modulated with the measurement frequency. This is most simply, and hence most frequently, done by modulating the supply current of the radiation source. A modulation of this kind, for a fiber-optical temperature sensor, is known from published British patent application GB-PA No. 21 13 837, for example.
However, the accuracy of measurement in frequency-proportional optical sensors is limited by the following properties of the radiation source:
(a) nonlinearity of the relationship between the supply current and the radiation output; PA1 (b) temperature dependency of the radiation output at a constant current; PA1 (c) time dependency of the radiation output at a constant current over short periods of time, caused for example by self-heating due to the operating current; PA1 (d) time dependency of the radiation output at a constant current over long periods of time caused by aging of the radiation source; PA1 (e) wavelength dependency of the properties (a) to (d) above; PA1 (f) alteration of the frequency response of the modulation amplifier and radiation source, if modulation or amplification varies.
and,